The sensing system of an electronic input scanner detects intensity of light imagewise derived from an original image over a selected integration period, and for a discrete area viewed, assigns sensed light intensity a representative analog value ranging over a continuous scale of values. Thus, the output signal of the electronic input scanner is a function representing intensity of light from the image, whether reflected from or directed through the image. To obtain an optimum representation of the scanned image, the image acquisition system must be carefully focused. Focusing entails careful control of the relative positions of the original image on a document or other surface, the photosite or array of photosites detecting light from the original image, and the lens which directs light reflected from the document to the photosites. A number of factors can vary the focus of the system, including improper placement of the lens with respect to the original or the photosite, incorrect or imperfect placement of the original image with respect to the lens, or incorrect placement of the photosites. Real causes of these imperfections might be folds in a document, a warped supporting platen, an array of photosites that sags along the array, an array improperly mounted with respect to lens and document, a lens that changes in optical characteristics due to heat, etc. All these potential threats to focus must be reviewed, and may be adjustable to vary the total conjugate of the system, the front conjugate or the rear conjugate.
Present arrangements for achieving a selected focus assume correct manufacture and design. Lens/photosite array combinations are available as unitary constructs, thereby setting a fixed rear lens conjugate, for which a proper front conjugate may be defined in the design phase of a device. However, actual manufacturing tolerances affect these conjugates and thus may cause an out of focus condition. To determine focus of a system, a measurement referred to as CTF (Contrast Transfer Function) is sometimes used. In this measurement, a square wave signal is provided as an input to the optical system, and the output of the system is directly measured for the maximum and minimum responses. The measurement assumes that for each cycle of a square wave which has a positive and negative half cycles, at the midpoint of each half cycle, a maximum or minimum respectively should occur, respectively, because the output function should track the input function. CTF is defined as: ##EQU1## CTF is related to the modulation transfer function (MTF) of the system, but employs a less expensive test pattern for measurement.
Signal to noise ratio is also a factor in the quality of image reproduction from electronic input scanners. Many operational elements of electronic input scanners introduce noise into the output signal of the system, i.e., signal components added to the output signal which do not reflect the intensity function, commonly including elements which operate unpredictably over time. Variations in illumination of the image over time, variability in operation of photosites over time, or inappropriate operation of a converter for the conversion of analog signals derived from the photosites to digital signals for image processing, are all sources of noise. Signal to noise ratio is defined as ##EQU2##
Typically measurements of either focus or signal to noise ratio are laboratory or bench tests requiring a testing apparatus, calibrated test patterns, and experienced test technicians. These measurements are impractical to make once the machine has been placed at a customer location, without removing the machine. These tests, while comprehensive and analytical, are also very time consuming and subject to error because of their complexity.
U.S. Pat. No. 4,123,778 to Graf demonstrates a determination of sharpness of an image through the evaluation of the absolute values of density differences between adjacent points of the image in at least one selected area of the image. Uniformity of printer operation is sometimes tested by supplying a ramp function to the printer, applying a threshold to the ramp, and observing the varying output result, which ideally should provide a white level converting into a black level when the ramp function value crosses the threshold.